SINKS FOR 15N-ENRICHED ADDITIONS TO AN OAK FOREST AND A RED PINE PLANTATION

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Abstract

We added 15N tracers to reference plots (receiving ambient N inputs) and to chronically fertilized plots (50 kg NH4NO3-N·ha−1·yr−1 for 3 yr prior to and during tracer additions) in an oak-dominated deciduous forest and a red pine plantation in order to quantify sinks for N inputs to these forests. Plots (30 × 30 m) were located at the Harvard Forest Long-Term Ecological Research (LTER) site in central Massachusetts. Two forms of 15N tracer were applied, with 15NH4 and 15NO3 added to separate halves of reference and chronically fertilized plots in each forest. Tracers were applied monthly during two growing seasons in order to simulate movements of background (8 kg·ha−1·yr−1) and chronically elevated (58 kg·ha−1·yr−1) N deposition. Forest floors and soils were the dominant sinks for N deposition in these forests, but the relative importance of trees as sinks for N inputs increased with N loading rate. Accumulations of 15N in tree tissues after 2 yr of tracer additions showed that tree leaves, fine roots, bark, and recently formed wood assimilated <5% of the 15N added to reference plots and 20–24% of the 15N added to fertilized plots. The form of N input influenced its movement into ecosystem pools. Percent recoveries of 15N in trees were typically greater after 15NO3 additions than after 15NH4 additions. Woody tissues (wood ≤5 yr old plus bark) accumulated small fractions of N inputs after 2 yr of tracer additions, with 15N recoveries of <1% under ambient N deposition and <5% under chronic N fertilization. Regional and global assessments of the effects of N deposition on forest carbon balances should take into account observations suggesting that, although the proportion of N deposition assimilated by trees could increase with N input rate, most N deposition retained by temperate forests is likely to accumulate in soil pools with low C{:}N rather than in woody biomass with high C:N ratios.

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